During the manufacture of a semiconductor device a semiconductor wafer section, such as a single die or a plurality of singularized or unsingularized die, is often attached to a support such as a lead frame. Methods of attaching the wafer section to the lead frame include backside attach and "leads over chip" (LOC). With backside attach a back (noncircuit) side of the wafer section can be attached to a paddle of the lead frame. Various materials are used for attaching the semiconductor die to the lead frame, for example thermosets, thermoplastics, eutectics or other metals, epoxies, tapes, or other workable materials. With LOC attach, leads of the lead frame are attached to a circuit side of the die thereby eliminating the paddle. An advantage of LOC over backside attach is that LOC allows for a larger die size in the same package footprint.
With conventional LOC attach a piece of LOC tape is applied between the circuit side of the wafer section and the lead frame. The LOC tape can comprise an inert carrier with a polymer on either side to mechanically interconnect the lead frame and the wafer section. LOC tape is conventionally purchased by a semiconductor assembly plant from a supplier, and is usually supplied in pre-cut and layered with polymer. The ideal size of the tape and amount of polymer, however, is dependent on the size of the wafer section. With a smaller wafer section tape of smaller dimensions is desirable which increases package reliability performance such as temperature cycle performance. Using LOC tape which has an overabundance of polymer can cause the device to fail during a solder reflow step due to absorption of moisture in a failure known as "popcorn crack defect mechanism." In addition, too much polymer can cause coefficient of thermal expansion (CTE) mismatch between the lead frame, the LOC polymer, the silicon wafer section, and a mold compound which encases the wafer section which can also lead to device failure.
To reduce the size of the LOC tape it must be reprocessed by punching or slitting the tape before it is rolled onto a spool which can be difficult with current manufacturing techniques. Sizing the tape as it is originally produced by the supplier can also be difficult as wafer section size can change depending on the process used to manufacture the wafer section and the type of device which is being produced.
If other materials are used for attaching the die to the lead frame the quantity of adhesive and the final thickness of the adhesive between the die and the lead frame must be carefully controlled. This thickness is conventionally controlled by dispensing a measured quantity of adhesive onto the lead frame or wafer section, then applying a controlled pressure for a timed interval to the die by the die attacher. The thickness of the adhesive between the die and the lead frame (the "bond line") is difficult to control in this manner, and can vary greatly with small variations in the viscosity of the adhesive, application temperature, and amount of applied adhesive. If an excessive amount of adhesive is applied, the adhesive can bleed out from under the die and prevent, for example, bond wires from properly attaching to lead fingers of the lead frame. If a uniform bond line is not achieved, the die will not be coplanar with the lead frame, which is known to have associated problems.
A method and apparatus for attaching a wafer section to the lead frame which reduces the problems described above would be desirable.